Room: Karl Dean Ballroom A2
Purpose: To develop a structure template for the Mentor CPX4 breast tissue expander metal port for proton pencil beam scanning (PBS) treatment planning and to evaluate the dose calculation accuracy of the treatment planning system (TPS).
Methods: A phantom with a breast tissue expander metal port immersed in 240ml of water in a plastic container on top of acrylic slabs was scanned using a Siemens Somatom Definition AS Open RT CT scanner (Siemens Medical, Erlangen, Germany). A structure template of the metal port was constructed based on the device geometry. The Hounsfield units for the metal port, plastic container with water, and the acrylic slabs were overridden based on measured relative stopping powers. A cylindrical CTV was used as target, and the prescription dose was 2Gy(RBE) in one fraction. A single-AP-beam plan and a two-oblique-beam plan were optimized using Eclipse TPS (Varian Medical System, Palo Alto, California). Forward dose calculations were also performed with 1cm isocenter shifts, mimicking the worst case of setup uncertainty, and the resulted dose degradations were evaluated. The calculated doses were compared to measured doses using Gafchromic EBT3 films at different depths in acrylic slabs.
Results: Comparison between the calculated and measured doses showed that the TPS underestimated the dose inhomogeneity distal to the metal port. The dose difference depended on the depth of the measurements and the beam arrangements. More than 15% dose difference was observed at different depths for both plans. Both calculated and measured doses showed that the displacement of the metal port by 1cm could cause the cold spots as low as 50% of the prescription dose in the target.
Conclusion: TPS underestimated dose inhomogeneity and the metal port setup uncertainty caused cold spots in the target. Such effect should be considered if PBS is used to treat patients with tissue expanders.